Switching power source apparatus

ABSTRACT

A power-factor improving circuit applicable to a switching power source apparatus includes a rectifier having a diode bridge, a step-up reactor, a switching element, an output diode, a smoothing capacitor, a controller, and a variable reference voltage generator. If an error voltage from a conductance amplifier exceeds a second reference voltage, a comparator provides a high-level switching signal to turn on a switching element and bypass a resistor. As a result, a first reference voltage supplied to the conductance amplifier is switched from a low reference voltage to a high reference voltage. This results in adjusting an output voltage (Vout) to a low value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching power source used for, forexample, electronic equipment, and particularly, to a switching powersource having a power-factor improving function to improve conversionefficiency.

2. Description of the Related Art

A switching power source apparatus having a power-factor improvingconverter is used for electronic equipment such as AC adaptors, OAequipment, and consumer equipment, and therefore, must be in compliancewith harmonic current regulations (IEC/EN6 1000-3-2) and harmonicsuppression guidelines for household and general-purpose appliances.There is a requirement in recent years for a high-efficiency switchingpower source apparatus capable of contributing to the downsizing andenergy-saving of electronic equipment.

FIG. 1 shows an example of a step-up chopper conforming to the harmoniccurrent regulations and serving as a switching power source apparatus.In FIG. 1, a power-factor improving converter consists of a rectifier 2employing a diode bridge, a switching element 4 turned on and off by acontroller 8, and a step-up reactor 3. This step-up chopper controls anON/OFF switching operation of the switching element 4 so that a peakcurrent of the step-up reactor 3 may follow an input voltage and providea constant output voltage conforming to the harmonic currentregulations.

SUMMARY OF THE INVENTION

To improve a power factor, the above-mentioned step-up chopper conductsa step-up operation such that an output voltage of the step-up chopperkeeps the following relationship with respect to the maximum value(multiplied by √2) of an input AC voltage:(output voltage Vout)≧(AC input voltage Vin)×√2  (1)

Namely, the output voltage must be increased if the input AC voltage Vinis increased.

If the input AC voltage is in the range of 90 Vac to 265 Vac, the outputvoltage of the step-up chopper must be “maximum AC input voltage×√2”such as 370 Vdc to 400 Vdc to properly conduct a power-factor improvingoperation even if the input AC voltage reaches the maximum value.

If the input AC voltage is low, for example, 90 Vac, the step-up choppermust increase this voltage to 370 Vdc to 400 Vdc due to the power-factorimproving operation. This involves a large step-up ratio. As the step-upratio increases, the switching element of the step-up chopper causes alarger loss to decrease the power conversion ratio of the step-upchopper.

It is strongly required, therefore, to realize a circuit capable ofconducting constant-voltage control on a low-voltage-range AC powersource of, for example, a 100-volt-based input voltage Vin, to providean optional constant output voltage Vout in the range of 230 Vdc to 250Vdc.

The present invention provides a switching power source apparatuscapable of improving conversion efficiency without increasing apower-factor improving step-up ratio.

According to a first technical aspect of the present invention, there isprovided a switching power source apparatus for ON/OFF-controlling aninput AC voltage with a switching element and converting the input ACvoltage into a DC voltage that is higher than the AC voltage and issupplied as an output voltage. The switching power source apparatusincludes an error amplifier configured to provide an error voltagerepresentative of a difference between the output voltage and a firstreference voltage, a controller configured to control the switchingelement according to the error voltage, the controller turning off theswitching element when a predetermined ON time is reached, and avariable reference voltage generator configured to generate the firstreference voltage according to the error voltage.

According to a second technical aspect of the present invention, thevariable reference voltage generator includes an error voltagecomparator configured to compare the error voltage with a secondreference voltage, and if the error voltage is greater than the secondreference voltage, provide a switching signal, and a reference voltageswitch configured to switch the first reference voltage to a lowerreference voltage in response to the switching signal and provide theerror amplifier with the lower reference voltage.

According to a third technical aspect of the present invention, thevariable reference voltage generator includes a voltage/impedanceconverter configured to change an impedance according to the errorvoltage and a voltage generator configured to generate the firstreference voltage, which decreases as the error voltage increases,according to the impedance provided by the voltage/impedance converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a switching power source apparatus according toa related art;

FIG. 2 is a view showing a power-factor improving circuit applicable toa switching power source apparatus according to a first embodiment ofthe present invention;

FIG. 3 is a view showing the details of the power-factor improvingcircuit of FIG. 2;

FIGS. 4( a) to 4(f) are timing charts explaining operation of theswitching power source apparatus according to the first embodiment ofthe present invention;

FIG. 5 shows waveforms of parts of the switching power source apparatusaccording to the first embodiment of the present invention;

FIG. 6 is a graph showing dependence between an error voltage Ver and aninput AC power source voltage under a rated load;

FIG. 7 is a graph showing a relationship between an error voltage Verand a first reference voltage;

FIG. 8 is a graph showing a relationship between output power and thefirst reference voltage;

FIG. 9 is a view showing a power-factor improving circuit applicable toa switching power source apparatus according to a second embodiment ofthe present invention; and

FIG. 10 is a view showing a continuous change in a first referencevoltage relative to an error voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will be explained with reference tothe drawings.

First Embodiment

FIG. 2 is a view showing a power-factor improving circuit applicable toa switching power source apparatus according to the first embodiment ofthe present invention. With reference to FIG. 2, the power-factorimproving circuit will be explained.

The power-factor improving circuit includes a rectifier 2 employing adiode bridge, a step-up reactor 3, a switching element 4, an outputdiode 5, a smoothing capacitor 6, a controller 8, and a variablereference voltage generator 50. The variable reference voltage generator50 includes an error voltage comparator 51 and a reference voltageswitch 52. The power-factor improving circuit is connected to a DC-DCconverter 9 that supplies a DC voltage to a load 10.

In FIG. 2, an AC power source 1 supplies a sinusoidal voltage to therectifier 2, which full-wave-rectifies the sinusoidal voltage andsupplies the rectified voltage to the step-up reactor 3 and controller8.

The step-up reactor 3 has a primary winding 3 a and a criticalitydetection winding 3 b. An end of the primary winding 3 a is connected toan output terminal of the rectifier 2, and the other end thereof isconnected to a drain of the switching element 4 and an anode of theoutput diode 5. An end of the criticality detection winding 3 b isconnected to the controller 8, and the other end thereof is connected toa ground GND. A cathode of the output diode 5 is connected to an end ofthe smoothing capacitor 6, an input terminal of the DC-DC converter 9,and the controller 8.

A gate of the switching element 4 is connected so as to receive a drivesignal from the controller 8. A source of the switching element 4 isconnected to the ground GND.

The power-factor improving circuit provides a DC output Vout to theDC-DC converter 9. The DC-DC converter 9 is, for example a flybackconverter to convert the DC voltage provided by the power-factorimproving circuit into another DC voltage, which is applied to the load10.

The controller 8 has an error amplifier. The error amplifier provides anerror voltage comparator 51 with an error voltage representative of adifference between the output voltage from the power-factor improvingcircuit to the DC-DC converter 9 and a first reference voltage. Theerror voltage comparator 51 compares the error voltage provided by theerror amplifier of the controller 8 with a second reference voltage, andif the error voltage is greater than the second reference voltage,supplies a switching signal to the reference voltage switch 52. Inresponse to the switching signal from the error voltage comparator 51,the reference voltage switch 52 switches the first reference voltage toa lower value and supplies the same to the controller 8.

FIG. 3 is a view showing the details of the power-factor improvingcircuit applicable to the switching power source apparatus according tothe first embodiment of the present invention. With reference to FIG. 3,the power-factor improving circuit will be explained.

In the controller 8, a non-inverting input terminal “+” of a comparator29 receives a voltage generated by the criticality detection winding 3b. If this voltage is lower than a reference voltage 28 (Vrefc), thecomparator 29 provides a low-level signal to a reset terminal of an RSflip-flop 30 and an input terminal of a NOR circuit 31. Then, the NORcircuit 31 provides a high-level signal to the switching element 4 toturn on the switching element 4.

Elements incorporated in the controller 8 will be explained. Aconductance amplifier (error amplifier) 23 generates an error voltageVer representing a difference between a voltage obtained by dividing theoutput voltage Vout by resistors 21 and 22 and a reference voltage Vref1provided by the reference voltage switch 52. A constant current source25 supplies a charge current to a capacitor 26. A comparator 27 comparesa terminal voltage of the capacitor 26, which increases according to thecharge voltage supplied by the constant current source 25, with theerror voltage Ver and determines an ON-time. An OR circuit 33 turns on aswitch 34 according to an output signal from the comparator 27, to resetthe terminal voltage of the capacitor 26. These elements function as anON-time determination circuit to determine an ON-time of the switchingelement 4. When the determined ON-time is reached, the comparator 27provides a set signal to the RS flip-flop 30 to set a Q-output terminalof the RS flip-flop 30 to a high level. Then, the NOR circuit 31provides a low-level signal to the switching element 4 to turn off theswitching element 4. This control stabilizes the output voltage Vout.

A filter 40 includes a resistor 41, a capacitor 42, and an operationalamplifier 43. The resistor 41 and capacitor 42 are connected to anoutput terminal of the operational amplifier 43. The resistor 41 andcapacitor 42 are set to provide a time constant corresponding to afrequency lower than a switching frequency of ON/OFF control of theswitching element 4. The operational amplifier 43 involves a voltagefollower connection. The error voltage Ver from the comparator 23 of thecontroller 8 is noise-reduced with a capacitor 49 and is supplied to anon-inverting input terminal (+) of the operational amplifier 43. Theoperational amplifier 43 provides an impedance-reduced error voltageVer, which is delayed by the resistor 41 and capacitor 42 and issupplied to a comparator 59.

The comparator 59 is in the error voltage comparator 51. The delayederror voltage Ver from the filter 40 is supplied to a non-invertinginput terminal “+” of the comparator 59, and a reference voltage Vref2from a variable reference voltage 58 is supplied to an inverting inputterminal (−) of the comparator 59. The comparator 59 compares thesevoltages with each other, and if the error voltage Ver is greater thanthe reference voltage Vref2, provides a high-level switching signal tothe reference voltage switch 52.

The reference voltage switch 52 includes a reference voltage source 53for generating a reference voltage Vref3 and a potentiometer (54 to 57)being connected to the reference voltage source 53. The potentiometerprovides an output voltage serving as the reference voltage Vref1. Thepotentiometer consists of resistance elements including a variableresistance element. The variable resistance element has a resistancevalue corresponding to a control signal provided by the error voltagecomparator 51. According to this embodiment, the potentiometer iscomposed of resistance elements 54 to 56. The resistance element 56 anda switching element 57, which is connected in parallel to both ends ofthe resistance element 56, form the variable resistance element. Theresistance element 56 whose one end is grounded is selectively bypassedunder the control of the switching element 57, to change the outputvoltage Vref1 of the potentiometer accordingly.

The switching element 57, i.e., a transistor (FET) 57 is turned on inresponse to a high-level switching signal from the error voltagecomparator 51. If a low-level signal is supplied to a gate terminal ofthe FET 57, the FET 57 is OFF. In this case, the reference voltage 53 isdivided by the resistors 54, 55, and 56 to provide a reference voltageVref1-1, which is provided as the reference voltage Vref1 to thenon-inverting input terminal (+) of the comparator 23. When a high-levelswitching signal is applied to the gate terminal of the FET 57, the FET57 is ON so that a drain of the FET 57 is connected to a source of theFET 57. In this case, the reference voltage 53 is divided by theresistors 54 and 55 to provide a reference voltage Vref1-2, which isprovided as the reference voltage Vref1 to the non-inverting inputterminal (+) of the comparator 23. In this way, the potentiometer ofthis embodiment can select one of the two voltage divided values madefrom the reference voltage Vref3 and can output the selected one as thereference voltage Vref1. The variable resistance element may be set atany one of the resistance elements of the potentiometer.

The reference voltages Vref1-1 and Vref1-2 have the followingrelationship:Vref₁₋₁>Vref₁₋₂  (2)

With reference to a timing chart of FIG. 4 and graphs of FIGS. 5 to 8,operation of the switching power source apparatus according to the firstembodiment of the present invention will be explained. The AC powersource 1 applies power to the power-factor improving circuit. Namely,the AC power source 1 supplies a sinusoidal voltage to the rectifier 2,which full-wave-rectifies the supplied voltage and provides afull-wave-rectified waveform to the power-factor improving circuit.

(1) Initial Operation

At first, the non-inverting input terminal (+) of the comparator 29 isconnected to the ground GND through the criticality detection winding 3b, and the inverting input terminal (−) of the comparator 29 receivesthe reference voltage Vrefc. The comparator 29 compares the inputvoltages with each other. Since the voltage at the non-inverting inputterminal is lower than the other, the comparator 29 provides a low-levelsignal to the RS flip-flop 30.

The RS flip-flop 30 is reset in response to the reset signal from thecomparator 29. At timing t1 of FIG. 4, the NOR circuit 31 provides ahigh-level drive signal to turn on the switching element 4.

When the switching element 4 is turned on, a drain voltage Vd of theswitching element 4 drops close to 0 V at the timing t1 of FIG. 4. Therectifier 2 passes a switching current to the primary winding 3 a, tothe drain and source of the switching element 4, and to the ground GND.As a result, the step-up reactor 3 accumulates energy.

(2) Operation of ON-Time Determination Circuit

At this time, in the ON-time determination circuit for determining anON-time of the switching element 4, the constant current source 25supplies a constant charge current to the capacitor 26. A terminalvoltage of the capacitor 26 gradually increases in proportion to time,to increase a voltage at the non-inverting input terminal of thecomparator 27.

The comparator 27 compares this voltage with an error voltage Versupplied to the inverting input terminal thereof. If the terminalvoltage of the capacitor 26 is greater than the error voltage Ver, thecomparator 27 provides, at timing t2 of FIG. 4, a high-level pulsesignal as an ON-time determination signal to a set terminal of the RSflip-flop 30 to set the RS flip-flop 30, as well as to the switch 34through the OR circuit 33 to turn on the switch 34. As a result, thecapacitor 26 discharges and is reset until the switching element 4 isturned on next time.

At this time, the Q-output of the RS flip-flop 30 is set to a highlevel. At the timing t2 of FIG. 4, the output of the NOR circuit 31provides a low-level drive signal to turn off the switching element 4 tostabilize the output voltage Vout.

(3) Input Current Waveform

The response characteristic of the error amplifier 23 is sufficientlyslow compared with a switching frequency, and therefore, the errorvoltage Ver connected to the inverting input terminal of the comparator27 of the ON-time determination circuit determining an ON-time of theswitching element 4 changes very slowly compared with a change in theterminal voltage of the capacitor 26 being connected to thenon-inverting input terminal “+” of the comparator 27.

Accordingly, it can be considered that an output signal from the ON-timedetermination circuit conducts switching control with a fixed ON-timeand a variable OFF-time in terms of one cycle of the frequency of the ACpower source 1.

When a voltage is applied to the step-up reactor 3, a current to thestep-up reactor 3 is determined by an energization time and the appliedvoltage. If control is conducted with a fixed ON-time and a variableOFF-time, a peak value of the AC power source 1 determines a current tothe step-up reactor 3. Accordingly, the current to the step-up reactor 3has a waveform similar to a voltage waveform of the AC power source 1.

A waveform A shown in FIG. 5 is a full-wave-rectified voltage waveformprovided by the rectifier 2, a waveform B is of the error voltage Verprovided by the conductance amplifier 23, and a waveform C is of theoutput voltage Vout provided by the output capacitor 6.

At the timing t2 of FIG. 4, the switching element 4 turns off. Then, theenergy accumulated in the step-up reactor 3 and the voltage supplied bythe rectifier 2 are combined to charge the output capacitor 6 throughthe rectifying diode 5. As a result, as shown in the waveform C of FIG.5, the output capacitor 6 receives a voltage that is higher than a peakvalue of the full-wave-rectified waveform supplied by the rectifier 2.

(4) ON-Control of Switching Element

When the discharge of the energy accumulated in the step-up reactor 3 isfinished, the criticality detection winding 3 b generates counterelectromotive force to invert the voltage of the criticality detectionwinding 3 b. This voltage is compared with the reference voltage (Vrefc)28 by the comparator 29, and at timing t3, the comparator 29 provides alow-level signal to the RS flip-flop 30 and NOR circuit 31.

In response to the signal from the comparator 29 and the Q-output of theRS flip-flop 30, the NOR circuit 31 provides a high-level output. As aresult, at the timing t3 of FIG. 4, a drive signal is again supplied tothe switching element 4 to turn on the switching element 4.

The operations mentioned above are repeated thereafter, to maintain aconstant output voltage at the output capacitor 6 of the power-factorimproving circuit. At the same time, a current from the AC power source1 shows a sinusoidal current waveform similar to the voltage waveform ofthe AC power source 1.

(5) Operation of Conductance Amplifier

When the drive signal to the gate terminal of the switching element 4changes to a low level to turn off the switching element 4, the terminalvoltage of the capacitor 26 connected to the non-inverting inputterminal of the comparator 27 increases. Namely, the terminal voltage ofthe capacitor 26 exceeds the error voltage Ver supplied to the invertinginput terminal of the comparator 27. The level of the error voltage Veris adjusted according to the state of the output voltage Vout.

Such level adjustment is carried out by the conductance amplifier (erroramplifier) 23. The conductance amplifier 23 receives a divided value ofthe output voltage Vout of the output capacitor 6 divided by theresistors 21 and 22 and compares it with the reference voltage Vref1. Ifthe resistor-divided value of the output voltage Vout is greater thanthe reference voltage Vref1, the conductance amplifier 23 provides alower error voltage Ver, and if the resistor-divided value of the outputvoltage Vout is smaller than the reference voltage Vref1, a higher errorvoltage Ver.

More precisely, if the resistor-divided value of the output voltage Voutof the output capacitor 6 is greater than the reference voltage Vref1,the error voltage Ver from the conductance amplifier 23 becomes smallerso that, in the ON-time determination circuit, a time of the terminalvoltage of the capacitor 26 connected to the positive input terminal “+”of the comparator 27 to reach the error voltage Ver connected to thenegative input terminal (−) of the comparator 27 becomes shorter.

If the terminal voltage of the capacitor 26 exceeds the error voltageVer, the comparator 27 provides a high-level pulse signal as an ON-timedetermination signal to the set terminal of the RS flip-flop 30. As aresult, the switching element 4 is quickly turned off to shorten anON-time and decrease the output voltage Vout.

If the charge voltage of the capacitor 26 to which a constant current issupplied exceeds the error voltage Ver, the ON-time determinationcircuit provides an ON-time determination signal to the RS flip-flop 30and discharges the capacitor 26 until the switching element 4 is turnedon. Namely, the ON-time of the switching element 4 can be determinedaccording to the magnitude of the error voltage Ver.

(6) Reference Voltage Vref1 Provided by Reference Voltage Switch 52

If the AC voltage Vin continuously increases from, for example, 90 V to250 V under rated power, the error voltage Ver provided by theconductance amplifier 23 follows a downward curve shown in FIG. 6.

The reference voltage Vref1 supplied to the non-inverting input terminal“+” of the conductance amplifier 23 is a variable reference voltage thatvaries according to the error voltage Ver provided by the conductanceamplifier 23. Namely, the error voltage Ver provided by the conductanceamplifier 23 is supplied through the filter 40 to the non-invertinginput terminal of the error voltage comparator 59. If the error voltageVer is above the reference voltage Vref2, the comparator 59 provides ahigh-level switching signal to put the FET 57 of the reference voltageswitch 52 in a conductive state. As a result, the reference voltageVref1 is switched from Vref1-1 to Vref1-2 as shown in FIG. 7.

Namely, if the error voltage Ver provided by the conductance amplifier23 is greater than the reference voltage Vref2, the reference voltageswitch 52 supplies the reference voltage Vref1-2 to the non-invertinginput terminal of the conductance amplifier 23 so that even a100-volt-based AC power source Vin can be stepped up to provide anoutput voltage Vout in the range of 230 Vdc to 250 Vdc as shown in FIG.8.

If the AC power source Vin is in a low range, e.g., a 100-volt-basedrange, the conductance amplifier 23 is operated with the referencevoltage Vref1-2, so that the output voltage Vout may keep an optionalconstant value within the range of 230 Vdc to 250 Vdc. This results inimproving conversion efficiency without increasing the step-up ratio ofthe power-factor improving circuit.

If the error voltage Ver provided by the conductance amplifier 23 isbelow the reference voltage Vref2, the reference voltage switch 52supplies the reference voltage Vref1-1 to the conductance amplifier 23so that even a 200-volt-based AC power source Vin can be stepped up toprovide a constant output voltage in the range of 370 Vdc to 400 Vdc asshown in FIG. 8.

If the AC power source Vin is in a high range, e.g., a 200-volt-basedrange, the conductance amplifier 23 is operated with the referencevoltage Vref1-1, so that the output voltage Vout may keep an optionalconstant value within the range of 370 Vdc to 400 Vdc. As a result, thepower-factor improving circuit can conduct a step-up operation toimprove a power factor.

The switching AC power source voltage may be within the range of 140 Vacto 170 Vac other than the 100-volt-based low range or the 200-volt-basedhigh range. In this case, even when the AC input power source Vinreaches a maximum value, the embodiment can step up the same to providea proper output voltage Vout while improving a power factor and powersource quality.

As shown in FIG. 7, the error voltage comparator 51 may be designed tohave a hysteresis characteristic between an ascending curve and adescending curve of the error voltage Ver. More precisely, a feedbackresistor 49 (not shown) may be connected between the positive inputterminal “+” and the output terminal of the comparator 59, so that thereference voltage is switched from Vref1-2 to Vref1-1 when the errorvoltage Ver decreases below the reference voltage Vref2, and fromVref1-1 to Vref1-2 when the error voltage Ver increases above thereference voltage Vref2+ΔV. This may prevent a malfunction due to noiseand the like.

The filter 40 arranged between the controller 8 and the error voltagecomparator 51 delays the error voltage Ver provided by the controller 8,to stabilize the constant voltage control.

(7) Characteristic Operation of this Embodiment

According to this embodiment, the error voltage Ver provided by theconductance amplifier 23 increases as the load increases, to drop theoutput voltage Vout. If the error voltage Ver exceeds the referencevoltage Vref2, the comparator 59 provides a high-level switching signalto the FET 57 to turn on the FET 57. As a result, the resistor 56 isbypassed, and the reference voltage Vref1 supplied to the conductanceamplifier 23 is decreased from Vref1-1 to Vref1-2.

When the reference voltage Vref1 is decreased, the error voltage Verprovided by the conductance amplifier 23 substantially becomesinvariable. As a result, the comparator 27 quickly provides a set signalto quickly turn off the switching element 4. This results in decreasingthe output voltage Vout. The constant voltage control is applicable to,for example, a 100-volt-based, low-voltage-range AC power source Vin, sothat the output voltage Vout may be set to an optional constant voltagein the range of, for example, 230 Vdc to 250 Vdc. In this way, it ispossible to improve conversion efficiency without increasing the step-upratio of the power-factor improving circuit.

Second Embodiment

FIG. 9 is a view showing a power-factor improving circuit applicable toa switching power source apparatus according to the second embodiment ofthe present invention. With reference to FIG. 9, the power-factorimproving circuit will be explained. In the following explanation, thesame parts as those of the first embodiment of FIG. 3 are representedwith the same reference marks and their explanations are omitted.

The second embodiment employs a variable reference voltage generator 50including a voltage/impedance converter 61 and a reference voltagegenerator 62.

The voltage/impedance converter 61 receives an error voltage Ver from afilter 40, converts it into an impedance according to the level of theerror voltage Ver, and provides a resistance value corresponding to thevoltage level.

The voltage/impedance converter 61 includes a variable impedance 66serving as a variable resistance element. Like the first embodiment,resistance elements 54 and 55 and the variable impedance 66 form apotentiometer.

Namely, in the reference voltage generator 62, a resistance valueprovided by the voltage/impedance converter 61 is connected to theresistor 55, to thereby form a variable composite resistor. Thecomposite resistor and the resistor 54 divide a reference voltage Vref3(53). The divided voltage is supplied as a reference voltage Vref1 to aconductance amplifier 23.

With reference to a graph of FIG. 10, a characteristic operation of theswitching power source apparatus according to the second embodiment ofthe present invention will be explained. The filter 40 provides an errorvoltage Ver to the voltage/impedance converter 61. According to thelevel of the error voltage Ver, the voltage/impedance converter 61converts the error voltage Ver into a resistance value corresponding tothe voltage level. The resistance value is supplied to the referencevoltage generator 62. In the reference voltage generator 62, theresistance value is connected to the resistor 55 in series, to form avariable composite resistor. The composite resistor and resistor 54divide the reference voltage 53, and the divided voltage is supplied asa reference voltage Vref1 to the conductance amplifier 23. If the errorvoltage Ver exceeds a reference voltage Vref2, the reference voltageVref1 given to the conductance amplifier 23 gradually continuouslydecreases from Vref1-1 to Vref1-2 according to the magnitude of theerror voltage Ver.

Namely, the voltage/impedance converter 61 generates a resistance valuethat decreases as the error voltage Ver increases, and the referencevoltage generator 62 generates the first reference voltage Vref1 thatdecreases as the resistance value provided by the voltage/impedanceconverter 61 decreases. Consequently, the first reference voltage Vref1that decreases as the error voltage Ver increases is used to conductconstant voltage control to decrease an output voltage Vout.

When the reference voltage Vref1 to the conductance amplifier 23gradually decreases, the error voltage Ver provided by the conductanceamplifier 23 becomes substantially invariable to gradually decrease theoutput voltage Vout. The constant voltage control is applicable to, forexample, a 100-volt-based, low-voltage-range AC power source Vin, sothat the output voltage Vout may be set to an optional constant voltagein the range of, for example, 230 Vdc to 250 Vdc. In this way, thesecond embodiment can improve conversion efficiency without increasingthe step-up ratio of the power-factor improving circuit.

Between a controller 8 and the voltage/impedance converter 61, a filter40 may be arranged to delay the error voltage Ver generated by thecontroller 8. This results in stabilizing the constant voltage control.

The first and second embodiments have been explained in connection witha critical current operation in which a reactor current is about 0 A.The present invention is not limited to such a case. The presentinvention is also applicable to a discontinuous operation in which areactor current is discontinuous and to a continuous operation in whicha reactor current is continuous.

The first and second embodiments have employed a flyback converter asthe DC-DC converter 9. The present invention is not limited to such acase. The present invention is also applicable to an RCC circuit, aforward converter circuit, a half-bridge circuit, a bridge circuit, andthe like.

Effect of Invention

The present invention sets a first reference voltage according to anerror voltage, to conduct constant voltage control on a low outputvoltage to improve conversion efficiency and power factor withoutincreasing a step-up ratio.

The present invention has no need of inputting an input voltage waveforminto a controller to make an input current waveform similar to the inputvoltage waveform. This eliminates the need of a multiplier for forming acurrent target value, thereby simplifying a circuit structure andreducing the number of connection terminals. Under heavy load, thepresent invention can decrease an output voltage for a low-voltage-range(100-volt-based) AC power source and can switch to a high output voltagefor a high-voltage-range (200-volt-based) AC power source. The presentinvention, therefore, can reduce the loss of a switching element and canimprove a power conversion ratio even with a low-voltage-range AC powersource. In addition, the present invention can carry out a power-factorimproving operation with any of the low- and high-voltage-range AC powersources.

When the first reference voltage is changed to another referencevoltage, a power source voltage substantially corresponds to an ACvoltage between the low and high ranges, and therefore, the presentinvention can stably conduct constant voltage control and improve apower source quality for any of the 100- and 200-volt-based AC powersources. In addition, the present invention can conduct a power-factorimproving operation at or around the power source voltage at which thefirst reference voltage is changed to the other reference voltage.

The present invention controls the first reference voltage with the useof an error voltage, to prevent a malfunction due to noise. The presentinvention needs no multiplier nor input voltage waveform to be inputinto a controller to make an input current waveform similar to an inputvoltage waveform. Accordingly, the present invention involves noincrease in the number of pins nor a package change when forming ICs.This results in suppressing costs and package size.

This application claims benefit of priority under 35USC §119 to JapanesePatent Applications No. 2004-100756, filed on Mar. 30, 2004, the entirecontents of which are incorporated by reference herein. Although theinvention has been described above by reference to certain embodimentsof the invention, the invention is not limited to the embodimentsdescribed above. Modifications and variations of the embodimentsdescribed above will occur to those skilled in the art, in light of theteachings. The scope of the invention is defined with reference to thefollowing claims.

1. A switching power source apparatus for ON/OFF-controlling an input ACvoltage with a switching element and converting the input AC voltageinto a DC voltage that is higher than the AC voltage and is supplied asan output voltage, comprising: an error amplifier configured to providean error voltage representative of a difference between the outputvoltage and a first reference voltage; a controller configured tocontrol the switching element according to the error voltage, thecontroller turning off the switching element when a predeterminedON-time is reached; and a variable reference voltage generatorconfigured to generate the first reference voltage according to theerror voltage, wherein the variable reference voltage generatorcomprises: an error voltage comparator configured to compare the errorvoltage with a second reference voltage, and adapted, if the errorvoltage is greater than the second reference voltage, to provide aswitching signal; and a reference voltage switch configured to switchthe first reference voltage to a lower reference voltage in response tothe switching signal and provide the error amplifier with the lowerreference voltage.
 2. The switching power source apparatus of claim 1,wherein the error voltage comparator has a hysteresis characteristic inconnection with the second reference voltage.
 3. The switching powersource apparatus of claim 1, further comprising a filter arrangedbetween the controller and the error voltage comparator and configuredto delay the error voltage.
 4. A switching power source apparatus forON/OFF-controlling an input AC voltage with a switching element andconverting the input AC voltage into a DC voltage that is higher thanthe AC voltage and is supplied as an output voltage, comprising: anerror amplifier configured to provide an error voltage representative ofa difference between the output voltage and a first reference voltage; acontroller configured to control the switching element according to theerror voltage, the controller turning off the switching element when apredetermined ON-time is reached; and a variable reference voltagegenerator configured to generate the first reference voltage accordingto the error voltage, wherein the variable reference voltage generatorcomprises: a voltage/impedance converter configured to change animpedance according to the error voltage; and a voltage generatorconfigured to generate the first reference voltage, which decreases asthe error voltage increases, according to the impedance provided by thevoltage/impedance converter.
 5. The switching power source apparatus ofclaim 4, further comprising a filter being connected between thecontroller and the voltage/impedance converter and configured to delaythe error voltage.
 6. A switching power source apparatus forON/OFF-controlling an input AC voltage with a switching element andconverting the input AC voltage into a DC voltage that is higher thanthe AC voltage and is supplied as an output voltage, comprising: anerror amplifier configured to provide an error voltage representative ofa difference between the output voltage and a first reference voltage; acontroller configured to control the switching element according to theerror voltage, the controller turning off the switching element when apredetermined ON-time is reached, wherein the controller turns off theswitching element if a charge voltage of a capacitor to which a constantcurrent is supplied exceeds the error voltage, and discharges thecapacitor until the switching element is turned on; and a variablereference voltage generator configured to generate the first referencevoltage according to the error voltage.
 7. A switching power sourceapparatus for ON/OFF-controlling an input AC voltage with a switchingelement and converting the input AC voltage into a DC voltage that ishigher than the AC voltage and is supplied as an output voltage,comprising: an error amplifier configured to provide an error voltagerepresentative of a difference between the output voltage and a firstreference voltage; a controller configured to control the switchingelement according to the error voltage, the controller turning off theswitching element when a predetermined ON-time is reached; and avariable reference voltage generator configured to generate the firstreference voltage according to the error voltage, wherein the variablereference voltage generator comprises: a variable resistance elementconfigured to vary a resistance value according to the error voltage;and a potentiometer including a first resistance element and a secondresistance element connected in series, configured to generate the firstreference voltage, an end of the first resistance element that is notconnected to the second resistance element being connected to a thirdreference voltage, an end of the second resistance element that is notconnected to the first resistance element being grounded, the variableresistance element being connected to a part of the second resistanceelement so as to form a series circuit.
 8. The switching power sourceapparatus of claim 7, wherein the variable resistance is a parallelcircuit including a third resistance element and a switching element. 9.The switching power source apparatus of claim 7, wherein the variableresistance element is a voltage/impedance converter to change animpedance according to the error voltage.
 10. A switching power sourceapparatus for ON/OFF-controlling an input AC voltage with a switchingelement and converting the input AC voltage into a DC voltage that ishigher than the AC voltage and is supplied as an output voltage,comprising: an error amplifier configured to provide an error voltagerepresentative of a difference between the output voltage and a firstreference voltage; a controller consisting of connections to acriticality detection winding, an output diode, a variable referencevoltage generator, and the switching element, wherein the controller isconfigured to control the switching element according to the errorvoltage, the controller turning off the switching element when apredetermined ON-time is reached; and the variable reference voltagegenerator is configured to generate the first reference voltageaccording to the error voltage.